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dystrophies of the teeth. 6

Histological Characteristics the Basis for Classification.
To one who has made careful histological studies of the
structure of the teeth, these various deformities point to the
need for further investigations. Knowing, as we do, the plan
on which the teeth are gradually formed from certain points of
beginning, we recognize one form of dystrophy in which all of
the hard structures of the various teeth that were in process of
formation at a given time were imperfectly formed, due to an
interruption of the normal activities of constructive cells. We
note that the portions of the teeth formed both before and after
the particular time are perfect. This suggests at once a gen-
eral interruption in the nutritive processes during the period of
malformation, and inquiry as to the health of the individual at
the age indicated by the malformed part reveals the relation-
ship between the two as cause and effect.
We recognize another form of dystrophy in which there is
a failure of the formation of the cementing substance between
the enamel rods in irregular areas, notwithstanding the fact that
all other structures of the same teeth, and the cementing sub-
stance in other parts of these teeth, formed during the same
period, are perfect. Or there may be a partial failure in the
development of the enamel rods, as seen in the pits in teeth
which are otherwise normal in their formation. Such conditions
represent an interruption of the normal activities of certain
elements of the formative cells, while cells of the same type
close by have functionated properly. These .contraindicate a
general systemic disturbance and speak for a purely local inter-
ference.
We observe another form of dystrophy in which the cement-
ing substance between the enamel rods is entirely wanting in
all of the teeth of an individual, while every enamel rod is per-
fectly formed. In this there apparently is a disturbance or lack
of activity of an entire group of formative elements, and again
we must think of the cause as being more general. We may
imagine the lack of a certain necessary stimulus, as a result of
which the cells which should form the cementing substance have
failed to do so. Although no such relationship is known, the
situation is comparable to the relationship between certain duct-
less glands, as the thyroid and suprarenal glands, and other
organs and cellular elements, the secretion from these glands
in normal quantity being necessary for the proper functional
activity of the related organs or cells.
Again we see in the mottled teeth an endemic type of dys-
trophy, confined to persons living in certain geographic areas.